In the treatment of solutions that contain one or more elements in cationic, anionic or radical form dissolved therein, a separation of one or more ions or radicals can be accomplished by using electrodialysis. In electrodialysis, a feed solution is circulated through an electrodialyzer cell that comprises an anode and a cathode positioned in an anode and a cathode compartment, separated from each other by a multiplicity of alternating anionic and cationic selective exchange membranes. The alternating membranes and associated gaskets in appropriate arrangement form alternating concentrate and diluate cells. A direct current is applied between the electrodes and solution is fed to the diluate cells. A concentrate enriched in the ions that have passed through the membranes is withdrawn from the concentrate cells.
Electrodialysis has been used in a multitude of applications which include the separation of cations, anions, monovalent from multivalent ions, ionic species of the same or different electrical charge, acids from elements in ionic form, amphoteric elements, compounds of different conductances and degree of dissociation in solution, and ionic species from nondissociated organic compounds.
The removal of ionized species by electrodialysis is well documented but few references deal with the separation of ionized species from nonionized species. According to U.S. Pat. No. 2,854,393, electrodialysis is used for the fractionation of cations, anions, organic and inorganic ions, the separation of nonionic and nondissociated compounds in solution, as well as the separation of different compounds of different conductances and different degrees of dissociation in solution. Examples of separations of nonionic compounds all relate to organic compounds. Separation of compounds of different degrees of dissociation are only mentioned in passing, and relate to aqueous solutions of sodium chloride, boric acid, sodium acetate and acetic acid. All these fractionations and separations are carried out, however, with the application of a mechanical or gravitational accelerating force resulting in the formation of separate fluid strata in the cell from which solutions are withdrawn that contain an increased concentration of one of the species to be separated.
The separation of chloride ions and impurities from boric acid by electrodialysis has been discussed by Russian authors (Chem. Abs., vol. 60, 11404b and vol. 78, 140772p). The separation of calcium ions from phosphoric acid by electrodialysis is disclosed in Belgian Patent 643 464. Japanese unexamined application 79 132 496 discloses electrodialysis of an aqueous sodium silicate solution at pH 1-3 and containing 3-25% SiO.sub.2 together with 0.5N sodium chloride solution. Sodium ions and sulfuric acid were removed from a silica sol.
It appears from these references that separations of ionized from nonionized species by electrodialysis has only been carried out with a very limited number of elements.